The present invention relates to prostheses for replacement of a portion of the hip joint. More particularly, the present invention relates to a modular prosthesis for replacement of the upper portion of the femur.
Conventional prostheses for the replacement of the upper portion of the femur are generally unitary structures. The conventional prosthesis generally includes a stem portion that is designed to extend downwardly into a cavity within the femur. The stem portion may be secured within the femur by the use of bone cement, or in other adaptations, the stem may be configured to promote bone ingrowth to secure the stem. The conventional hip prosthesis also includes a neck portion that is integral with the stem portion. The neck portion is configured to replace the neck of the femur which will normally be resected with the head of the femur. Integrally fixed to the neck portion will normally be a generally spherical head portion that is configured to replace the head of the femur. A collar may be provided between the neck portion and the stem portion to stabilize the prosthesis on the resected femur. Generally, the collar will rest upon the resected surface of the femur to disperse the load on the prosthesis over a greater area, and to compress the underlying bone. A collar may be provided between the neck portion and the stem portion to stabilize the prosthesis and function as a stop or reference point on the resected femur.
Because conventional prostheses are generally unitary devices, the practice has been to maintain a large inventory of prostheses of different sizes to accommodate the different bone sizes that are to be replaced. Generally, the patient is evaluated by x-ray or some other means to determine the approximate bone size, and consequently the approximate required prosthesis size. During the replacement operation, several different prostheses are made available as suggested by the evaluation. The appropriate prosthesis is then selected for insertion into the patient at the time of operation.
One problem with the conventional, unitary prosthesis is that the hospital has to maintain a large number of differently sized prostheses to accommodate different sizes of bones to be replaced. Generally, a supply of prostheses with and without a collar also must be maintained in the inventory. Also, prostheses with different shapes and angles of the stem and neck portion must be maintained. Even with this large inventory of prostheses, it is often difficult to find a prosthesis that is sized and shaped exactly for the individual patient.
Prostheses having a stem portion and a separate head portion are known. One such prosthesis is shown in U.S. Pat. No. 4,051,559. U.S. Pat. No. 4,051,559 discloses a prosthesis that includes a separate threaded stem portion that is adapted to be screwed into a prepared cavity within the femur. The prosthesis separately comprises a head portion that includes a neck and collar that is adapted to be mated with the stem portion. This prosthesis is not designed to be assembled prior to insertion within the patient. The stem portion must first be screwed into the cavity within the femur. The head portion is then attached to the installed stem portion by a bolt. The collar is designed to rest upon the resected surface of the femur to support the load placed on the prosthesis. Because of the design of the stem portion, the head portion must include the collar in order to support the weight that will be placed on the prosthesis. This prosthesis is limited in flexibility because the stem portion must be straight in order to be screwed into the femur. Also, the head portion, including the neck and collar is a unitary structure which further reduces the flexibility of the device.
Another prosthesis having a stem portion and a separate head portion is shown in U.S. Pat. No. 3,806,957. U.S. Pat. No. 3,806,957 discloses a prosthesis that includes a separate stem portion having a proximal end that is broadened somewhat. The broadened proximal end of the stem is configured to receive a head and threaded neck portion to form a complete prosthesis. The patent discloses that the neck may be elongated or shortened depending on the specific anatomy of different patients. This prosthesis, like the prosthesis disclosed in U.S. Pat. No. 4,051,559, is limited in flexibility because the head and neck portion is a unitary structure, and because the head and neck portion attaches directly into the stem portion.
A prosthesis in which the stem portion comprises more than one component is shown in U.S. Pat. No. 3,987,499. U.S. Pat. No. 3,987,499 discloses a prosthesis having a stem or shank component that includes two parts, an anchoring part and a transition part. A ball is connected to the transition part. Also, a collar may be included between the ball and a portion of the femur. The anchoring part is provided with external threads that are adapted to tap themselves into the femur. The transition part is coupled to the anchoring part by a guide pin and securing screw. The ball is adapted to be screwed onto the free end of the transition part. One problem with the prosthesis disclosed in U.S. Pat. No. 3,987,499 is that the prosthesis is designed to be placed in position within the body component by component, and assembled sequentially. Another problem with this prosthesis is that the neck is a part of the transition part, which reduces the flexibility of the device. In addition, the collar is configured to be supported only between the bone and the ball, and is thus subject to rotational and toggling instability.
One object of the present invention is to provide a modular hip prosthesis that has a great deal of flexibility in its assembly, both as to the size and shape of the assembled device.
Another object of the present invention is to provide a modular hip prosthesis that may be assembled in the operating room before any component is inserted into the patient.
Yet another object of the present invention is to provide a modular hip prosthesis that is configured to function with or without a collar.
Yet another object of the present invention is to provide a modular hip prosthesis that includes a stem portion that may be shaped to fit a curvature within the bone structure, if necessary.
According to the present invention, a kit for the assembly of a hip prosthesis for the replacement of a portion of a femur is provided. The kit comprises a stem member having an upper portion and a lower portion, with the lower portion sized to be received into the femur. The kit also comprises a body member that is sized to replace a portion of the femur and is configured to be received over the upper portion of the stem member. In a preferred embodiment, the body member includes a tapered longitudinal bore that is generally aligned with the longitudinal axis of the femur. The stem member has an upper portion received and secured in the tapered longitudinal bore of the body member. The upper portion of the elongated stem member preferably includes a tapered portion adapted to be snugly received in the longitudinal bore and a cylindrical portion contiguous to the lower portion received in the lowermost extremity of the longitudinal bore, the cylindrical portion relieving any stress concentration at the lower margin of the bore.
The kit also comprises a head member that is sized to replace the head of the femur, including means to attach the head member to an upwardly and inwardly directed portion of the body member. The means for engaging the head member with the upwardly and inwardly directed portion of the body member preferably comprises a neck member including a first end formed to include a tapered projection that is sized to be received into a tapered hole in the head member to secure the head member to the neck member, and a second end for engaging an opening in the upwardly and inwardly directed portion of the body member.
In a preferred embodiment the neck member includes a threaded portion and a tapered portion for engaging threaded and tapered portions of the opening in the body member. The first end of the neck member preferably includes a frangible torque control means for controlling the torque applied to the neck member when engaging the opening. The tapered portion of the tapered portion of the neck member first end has a taper angle sufficiently small to provide a wedging engagement, while the tapered portion of the neck member second end has a taper angle sufficiently large to avoid a wedging engagement.
One feature of the present invention is that the kit preferably consists of stem members, body members, and head members, and attaching means all of various sizes and shapes. These separate components are adapted to be assembled together to form a custom prosthesis of a desired size and shape. One advantage of this feature is that a hip prosthesis of a desired size and shape may be assembled from the kit at the time of the operation.
Another feature of the present invention is that the kit comprises body members that are inserted over the stem members and are configured to bear a portion of the load that is exerted on the prosthesis. One advantage of this feature is that the prosthesis may be assembled without a collar component if the indications presented during the operation suggest that a collar may not be needed, or may produce unsatisfactory side effects.
In preferred embodiments of the present invention, the stem member, body member, and the head member are selected from a group consisting of differently sized and shaped members, respectively. One feature of the foregoing structure is that with different sizes and shapes of stem members, body members, and head members, the number of combinations available to assemble a hip prosthesis is greatly increased, without increasing the number of inventoried components. One advantage of this feature is that the flexibility of assembling a hip prosthesis for an individual patient is greatly increased.
The modular hip prosthesis of the present invention thus provides the ability to assemble a custom prosthesis by selecting different sizes and shapes of individual components to meet the requirements of the individual patient exactly. The provision of the kit concept for the prosthesis greatly reduces the inventory required to be maintained by the hospital. Also, the kit form of the prosthesis increases greatly the flexibility of the system, and provides for the assembly of a prosthesis that may otherwise be unavailable.
Additional objects, features, and advantages of the invention will be apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived. The detailed description particularly refers to the accompanying Drawings.